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The official Keepers of Time will add a leap second to the world’s master clocks (in the U.S., that’s the U.S. Naval Observatory) on December 31 at 23:59:59 UTC. This extra second is necessary because official time depends on two timescales—one that uses atomic clocks and another that is dependent on the earth’s rotation—and they don’t match up perfectly.

Atomic clocks (that’s a NIST atomic clock on the left) use the internal resonance frequency of atoms to measure time. The atoms generate pulses at regular intervals. Count the pulses, and you have a clock that is constant and very accurate.

Earth’s rotation is the traditional form of timekeeping. It is what defines a day. However, while we call a day 86,400 seconds, it is really 86,400.02 seconds. All those .02 seconds add up over time. In addition, the earth’s rotation is not constant (it has been slightly slowing, and 900 million years ago a day was only 18 of our hours). Time as we know it changes.

To remedy the discrepancy between the two timescales, extra time is periodically added to the atomic clock; this is the 24th leap second since 1972.